D.c. modulation expander with u-shaped helices for parametric amplifiers



May 10, 1966 G. WADE 3,251,025

D.C. MODULATION EXPANDER WITH U-SHAPED HELICES FOR PARAMETRIC AMPLIFIERS I Jill!!! I Filed Sept. 16, 1959 IIVVE/VTOR QZen Wade %v44 AT UR/VEY United States Patent 3,251,025 .D.C. MODULATION EXPANDER WITH U-SHAPED HELICES FOR PARAMETRIC AMPLIFIERS Glen Wade, Menlo Park, Calif., assignor to Zenith Radio Corporation, a corporation of Delaware Filed Sept. 16, 1959, Ser. No. 840,336 2 Claims. (Cl. 330-43) The present invention pertains to transverse-mode beam type parametric amplifiers and is particularly directed to the structure of the modulation expander for establishing the requisite pumping field characteristically employed in such an amplifier. Inasmuch as the invention has special application to quadrupole type transverse-made parametric amplifiers, it represents a further development of the amplifying arrangement described and claimed in copending application Serial No. 747,764 filed in. the name of Glen Wade on July 10, 1958, and assigned to the assignee of the present invention, and this application is therefore a continuation-in-part of that application.

In a transverse-mode beam type parametric amplifier a stream of electrons projected along a path to a collecting anode is subjected to a focusing field which may conveniently be supplied by a solenoid encompassing the beam path. The strength of the focusing field is adjusted to the end that there is established a transverse electron resonance in the electron stream at a selected cyclotron frequency; in one particular case, this resonant frequency corresponds to the frequency of the signal to be amplified.

In order to impress the signal upon the electron stream, a coupler is located at one point along the beam path,

preferably adjacent to the electron source; For arrangements wherein the resonance frequency corresponds to the signal frequency, the coupler may take the form of a pair of deflection plates located on opposite sides of the beam path and coupled to the source of a signal to be amplified. In this environment, the application of a signal to the deflection plates, causes deflection modulation of the electrons of the stream and results in orbital electron motion. Specifically, the electrons are caused to traverse a helical path having a radius which represents the instantaneous amplitude of the signal conveyed by the beam.

As explained in the Wade application, there is an added unique advantage in this structure in that the energy trans- I fer bet-Ween the coupler and the beam is bi-directional so that while the desired signal energy is modulated on the a stream there is also a transfer of energy from the stream to the coupler. This energy given up by the stream to the coupler is the fast wave noise or other fast wave signal components carried on the beam as it enters the field of the coupler.

Amplification of the signal wave developed on the beam under the influence of the coupler is achieved by increasing the radius of the electron orbits and this is the function of a modulation expander positioned along the beam path after the input coupler in the direction of the collector anode. In the Wade application the modulation expander is a quadrupole electrode structure positioned coaxially of the beam path and energized by means of an external pump signal source which has a frequency, in

one mode of operation, corresponding to twice the cyclotron frequency. The resulting quadrupole field is symmetrical with respect to the beam path but is non-homogeneous, increasing in intensity with radial distance from the beam path. This alternating field may be resolved into two counter-rotating field components each revolving at the cyclotron frequency, an analogy to the more familiar field of rotating electrical machines. The field which rotates in synchronism with an electron as it traverses the expander aifects the radius of the orbit of that electron, either increasing or decreasing the radius depending upon the phase condition of the particular electron under consideration. For a favorable phase condition, the radius increases and amplification of the signal Wave is experienced; for adverse phase conditions, the radius is decreased and the signal wave is attenuated. As explained in the earlier application, amplification overrides attenuation giving 'a.net increase in radius of the electron paths and, therefore, amplification of the signal conveyed by the beam.

While this is a highly useful and satisfactory modulation expander, there are obvious advantages to be gained if the modulation expansion can be accomplished without the requirement of an external pumping signal source- This is true for high frequencyapplications to which the parametric amplifier is particularly suited becauseit is then desirable to obviate the need for a very high frequency pump-signal generator. This desired objective is obtained with the modulation expander of the subject invention.

Accordingly, it is a principal object of the invention to provide a novel modulation expander for a transversemode beam type parametric amplifier.

It is another object of the invention to provide a new and improved transverse-mode beam type parametric amplifier which does not require an external source for supplying an alternating pumping signal.

A modulation expander constructed in accordance with the invention is particularly useful fora transverse-mode beam type parametric amplifier in which electrons of a beam traverse a helical path of predetermined pitch and with a radius representing the amplitude of the signal conveyed by the beam. The expander comprises a multifilar field generating structure arranged coaxially of the beam. This structure has a plurality of pairs of spaceopposed conductive elements individually skewed to represent a helix which is co-directional with and of the same pitch as the helical path of the eletrons. Moreover, the field structure has an axial length corresponding to at least one complete convolution of the helix. A unidirectional source ,of potential is connected with one polarity to the even-numbered pairs of the conductive elements of the field structure and is connected with'opposed polarity to the odd-numbered pairs of the conductive elements of that structure. This excitation establishes within the expander a pumping field which is unidirectional in a time domain but which exhibits polarity reversals at spaced positions along the path to simulate a symmetrical nonhomogeneous pumping field rotated in synchronism with the electrons to expand the radius of their orbital motion.

In particular, the conductive elements individually are generally of U-shaped cross-section and are mutually oriented to spaced adjacent legs in side-by-side aligned, parallel and electrically insulated relationship with the bight portions disposed nearest to and facing the axis of the beam. ually of a cross-section, in a plane containing the beam axis, which is convex as viewed from the axis and the internal boundary of the field generating structure has a cross-section, in a plane transverse to the axis, which defines substantially a square.

. The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in-connection with the 1 accompanying drawing, in the several figures of which These bight portions advantageously are individ- 3 like reference numerals identify like elements, and in which:

FIGURE 1 is a schematic diagram of one form of fast wave transverse-mode beam type parametric amplifier including a modulation expander constructed in accordance with the invention;

FIGURE 2 is a sectional view of the modulation expander included in the amplifier of FIGURE 1; and

FIGURE 3 is an end view of the modulation expander including a representation of the pumping field established therein.

Referring now more particularly to FIGURE 1, the arrangement there represented is a transverse-mode beam type parametric amplifier which comprises a source or electron gun for developing and projecting a stream of electrons along a path or axis 11. The beam source may be entirely conventional and preferably includes a I cathode together with suitable focusing and accelerating electrodes for developing a well-defined beam of electrons. For convenience of illustration, this source has been represented by the usual symbol for an indirectly heated cathode followed by two apertured electrodes. Electron beam collector 12 is disposed at the end of the beam path remote from the cathode and takes the form of an anode biased at a positive potential with respect to the cathode as indicated by the symbol +B.

The amplifier has means for creating in the beam path a field for establishing transverse electron resonance in the beam projected along that path. While electron resonance may be established through the agency of a magnetic or an electrostatic field, the arrangement under consideration employs a solenoid 13 surrounding the beam path throughout most of its length to establish lines of magnetic flux parallel thereto. The solenoid is energized by any suitable D.C. source (not shown) and the strength of the focusing field is established by adjustment of the excitation potential to produce a selected cyclotron frequency for electron motion. The focusing field of the solenoid is indicated symbolically by the arrow H and will be understood to extend over so much of the path length as includes the inputand output couplers as well as the modulation expander to be described presently.

Also positioned along the beam path are means for modulating the electron stream in response to an applied signal frequency. This modulating means is an electron coupler for imparting energy to the beam in response to signal energy received from a source 16. Different forms of coupling structures may be employed. They may be resonant cavities, transmission lines or deflection plates spaced alongside the beam for interaction therewith. As illustrated the input coupler or modulator 15 includes a pair of deflector plates 17, 18 located on opposite sides of the beam path. For coupling signal source 16 to the modulator a transmission line 19 having one end shortcircuited is coupled at its opposite end to deflectors 17, 18. A transmission link 21 is tapped onto transmission line 19 in a position adjusted to match the impedance of source16 to that represented by deflectors 17, 18. T ransmission line 22 has an effective electrical length of onequarter wave length at the frequency of the signal from source 16.

As explained above, the electron signal wave of a beam type parametric amplifier is represented by orbiting electrons traversing a helical path with a radius rep-resenting the instantaneous amplitude of the signal and amplification is obtained by a modulation expander employed to effect -a net increase in radius of the electron motion. The modulation expander is a multi-filar field-generating structure that is arranged coaxially of beam path 11. It has a plurality of pairs of space-opposed conductive elements individually skewed to represent a helix which is co-directional with and of the same pitch as the helical path of the electrons. Additionally, the structure has an axial length corresponding to at least one but preferably several complete convolutions of the helix. More specifically, the field structure usually has an even-number of pairs of conductive elements which are generally multiples of four although this is not a necessary restriction on the device. As represented, it is a quadri-filar structure of four conductors 21-24 as shown in detail in FIG- URE 2. These conductive elements are identical in construction, taking the form of a helically shaped conductor of U-shaped cross-sectional configuration. They are interlaced to define a multi-filar field structure having a centrally extending aperture which accommodates the electron beam. Apart from the advantages noted in the next paragraph, forming the conductors of U-shaped cross section is merely for convenience of assembling them into a unitary Whole for support within the tube envelope. Insulation (not shown) is interposed between adjacent legs or flanges of contiguous conductive elements to make the structure mechanically rugged while preserving electrical isolation of each conductor from its immediate neighbors.

In practicing the invention in its simplest form, it is only necessary that the four conductors define a quadrifilar field structure with its individual conductors so isolated from the adjacent conductors that they may be maintained at desired relative potentials as explained presently. Ideally, the quadri-filar structure should have a cross section, viewed in a transverse plane all along its length, representing a quadrupole of four hyperbolically shaped electrodes. That configuration assures optimum symmetry of the pumping field and also provides the necessary non-homogeneity in that the field is zero or a minimum on the beam axis and increases with distance radially outward of that axis. A practical and useful approximation to this ideal is obtained when the con ductive elements are shaped so that the effective pole pieces of the structure, when viewed in cross section, define a square as represented in FIGURE 3. This end may be accomplished by having the bight of each U-shaped conductive element convex with respect to the beam axis as shown in FIGURE 2.

A unidirectional source of potential, shown as a battery 25, is connected with one polarity to the even-numbered pairs of conductors of the field structure and connected with opposed polarity to the odd-numbered conductor pairs of the structure. For the quadri-filar embodiment, conductors 21 and 23 are connected to the positive pole of the battery and conductors 22 and 24 are connected to the negative pole of the battery.

Beyond modulation expander 20 in the direction of collector 12 is an output coupler or demodulator 30 serving to extract amplified signal energy from the beam for application to a load 31. The output coupler is identical in structure to input coupler 15 and its component parts bear identical reference characters. I

As thus far described, with the exception of modulation expander 20, the parametric amplifier of FIGURE 1 is essentially the same as that disclosed in the earlier filed application. The structure, except for its potential sources, load 31 and solenoid 13, is enclosed within an evacuated envelope (not shown). In operation, an electron beam issued from source 10 enters the field of input coupler or modulator 15 wherein it is modulated with the signal from source 16. It may be assumed that the signal frequency is essentially the same as the transverse electron resonant frequency established by focusing field H. Interaction of the beam and input modulator 15 results in deflection modulation of the beam in accordance with the instantaneous amplitude of the signal from source 16. The deflection modulation in conjunction with the focusing field and the forward component of electron velocity causes the electrons to orbit in a helical path having a pitch determined by the electron velocity and focusing field and having a radius representing the instantaneous amplitude of the signal conveyed by the beam. At the same time, fast wave electron beam noise or other fast wave signal components carried by the beam into the field The field encountered at the leading edge of the modulation expander may be that represented by the equipotential lines '35 of FIGURE 3. It is a symmetrical quadrupole field and its effect on an entering electron is dependent upon the phase conditions. The circular path within the field represents the orbit traversed by the electrons at the cyclotron frequency and the two horizontal and the two vertical arrows, pointing respectively toward and away from the beam axis, show the forces exerted upon the electrons in the four regions or quadrants of the structure. The electrons represented by the filled smaller circles in the first and third quadrants encounter the forces indicated by the smaller arrows which accelerate them along their counter-clockwise path. Other electrons, shown as empty circles in the second and fourth quadrants, are subjected to forces which decelerate their orbital motion. It is to be noted that there is no field at all at the center of the modulation expander and that the field intensity increases linearly with distance from' the center or the beam axis. As a consequence, the forces exerted upon an orbiting electron are proportional to the radius of the circle in which it moves so that the radius must increase or decrease exponentially. Whether the change in radius is one of increase or decrease is determined by the phase condition as already explained. Electrons entering with a favorable phase experience an increase and electrons entering with an unfavorable phase experience a decrease in radius of their orbit.

Since structure 20 is excited by a unidirectional potential source, the field condition within the expander is unidirectional or unchanging in a time. domain but the field is not unidirectional in a space dimension. More particularly, the field pattern of FIGURE 3 prevails at the'leading edge of the modulation expander. If the field is examined in a succession of transverse planes at increasing distances from the leading edge of the expander, the angular orientation of the field will appear to shift progressively in the same direction as the helical orbits of the electron. This angular displacement of the field pattern in the direction along the beam path results from the fact that conductive elements 21-24 of the expander are helical rather than linear in configuration. Since the pitch of the individual helices of the several conductors corresponds or is matched to the pitch of the electron orbits, the field within the expandel simulates a symmetrical non-homogeneous pumping field nm -l'otating in synchronism with the electrons to expand the radius of their orbital motion. An electron which enters in the best phase condition with respect to the apparently rotating pumping field remains in that phase throughout its passage through the expander and a phase focusing process exists whereby an electron which enters with an intermediate phase is shifted either toward the best phase condition or toward the worst phase condition depending upon its entering phase. In these respects, the influence of the pumping field on the electrons is, in all material respects, the same as that of the component of the pumping field within the quadrupole modulation expander of the earlier filed application which rotates in synchronism with the electrons.

Since the axial length of the expander is sufficient to embrace several complete convolutions of the electron helices, the expander field which is unchangeable in time exhibits polarity reversals at spaced positions along the beam path. This of course follows inasmuch as the conductive elements of the field structure are helicoidal as required to simulate a rotating pumping field condition.

The beam as it leaves modulation expander 20, having experienced a net increase in radius of its electron orbits, enters demodulator 30 where the amplified signal is extracted and delivered to load 31. Maximum amplification is attained if the pitch of the helix corresponds precisely with the pitch of the orbiting electrons although some amplification is realized if this relationship is not satisfied exactly. I

Viewed from a method concept, the disclosed structure contemplates that at least two pairs of space-opposed field generating poles are interleaved along helicoidal loci. For the structure illustrated, the field gen 'erating poles are the surfaces of conductive elements 21-24 which define the boundaries of the pumping field to which the beam is subjected. The helicoidal loci are coaxial of the beam path, are co-directional with and of the same pitch as the helical paths of the electrons and, preferably, their coaxial lengths correspond to several convolutions of the electron orbits. The simulated rotating pumping field-is established by energizing one pair of the field generating poles with one polarity of D.C. excitation potential and energizing the opposite pair with'the opposite polarity D.C. potential. The mechanical arrangement may take the form of continuous helical conductive elements as represented or, if mechanical expediency requires, they may be segmented to constitute an iterative structure.

The expander illustrated and described has the distinct advantage of achieving a net increase in radius of the electron orbits as required to amplify the signal in a transverse-mode beam type parametric amplifier without the need for a high frequency external pumping signal source. It is appropriate now to comment more particularly on the noise properties of the described device..

In all beam type parametric amplifiers there is an idler wave developed on the electron beam as an incident to' the modulation expansion; its frequency is equal to the pump frequency minus the signal frequency. Because of this characteristic there can be a noise contribution to the signal channel it the electron beam, as it enters the field of the modulation expander, carries a noise component corresponding in frequency to the idler. That component, through this same quasi-heterodyning process, is converted in frequency to the signal frequency and its presence is manifest as noise. In arrangements in Which the energy required for amplification is supplied by a high frequency pumping signal source, high in com parison with the signal frequency, both the idler and signal waves developed on the electron beam are characterized as fast waves. Since the idler is a fast wave, signal components on the beam correspondingtdthe idler may be removed at the input coupler as described above to minimize if not completely eliminate this undesired noise contribution. That is why such devices have extremely low noise properties. The present arrangement, employing a D.C. pumping field, does not exhibit the same low noise properties.

Where the energy for amplification is supplied through a D.C. pumping field, the pump frequency is zero and therefore the idler while being equal in frequency to the signal Wave is a slow wave. The input coupler which extracts noise from the electron beam is able to purge the beam only of fast wave components and cannot remove slow wave signal components. Accordingly,

the structure under consideration does not strip signal components corresponding to the idler from the beam at the input coupler and it, as -a consequence, does not exhibit the same property of extremely low noise. It is, however, very well suited to the amplification of signals at all but the very lowest power levels. Particularly, since the device avoids the need for a high frequency external pump source it is admirably suited for such purposes as high efiiciency amplifiers.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

1 claim:

1. A modulation expander for a transverse-mode, Ibeam type parametric amplifier in which electrons of a beam projected along a predetermined axis traverse a helical path of a predetermined pitch and with a radius representing the amplitude of a signal conveyed by said beam, said expander comprising: a multi-filar field generating structure arranged coaxial'ly of said beam with said structure having a plurality of pairs of space-opposed conductive elements individually skewed to represent a helix which is co-directional with and of the same pitch as said helical path of said electrons and said structure having an axial'length corresponding to at least one complete convolution of said helix and with said conductive elements individually being generally of U-shaped cross-section and mutally oriented to space adjacent legs in side-by-side aligned, parallel and electrically insulated relationship and to dispose the bight portions nearest to and facing said axis; and a unidirectional source of potential connected with one polarity to the even-numbered pairs of said conductive elements of said structure and connected with opposed polarity to the odd-numbered pairs of said conductive elements to establish within said expander a pumping field which is unidirectional in a time domain but which exhibits polarity reversals at spaced positions along said axis to simulate a symmetrical, non-homogeneous pumping field rotating in synchronism with said electrons to expand the radius of their orbital motion.

2. A modulation expander for a transverse-mode, beam type parametric amplifier in which electrons of a beam projected along a predetermined axis traverse a helical path of a predetermined pitch and with a radius representing the amplitude of signal conveyed by said beam, said expander comprising: a quadri-filar field generating structure arranged coaxially of said beam with said structure having two pairs of space-opposed conductive elements individually skewed to represent a helix which is co-directional with and of the same pitch as said helical path of said electrons and said structure having an axial length corresponding to at least one complete convolution of said helix and with said conductive elements individually being generally of U-shaped cross-section and mutually oriented to space adjacent legs in side-by-side aligned, parallel and electrically insulated relationship and to dispose the bight portions nearest to and facing said axis, said bight portions individually having a cross-section,,in a plane containing said axis, which is convex as viewed from the axis and the internal boundary of said structure having a cross-section, in a plane transverse to said axis, which defines substantially a square; and a unidirectional source of potential connected with one polarity to one of said pairs of conductive elements of said structure and connected with opposed polarity to the other of said pairs of conductive elements to establish within said expander a pumping field which is unidirectional in a time domain but which exhibits polarity reversals at spaced positions along said axis to simulate a symmetrical, non-homogeneous quadrupole pumping field rotating in synchronism with said electrons to expand the radius of their orbital motion.

References Cited by the Examiner UNITED STATES PATENTS 5/1958 Kompfner 3153.6 2,959,740 11/1960 Adler 3305 OTHER REFERENCES Adler et al.: Proceedings of the IRE, October 1958, pp.1756-1757.

ROY LAKE, Primary Examiner.

BENNETT G. MILL'ER, ELI SAX, Examiners.

D. R. HOSTETTER, Assistant Examiner. 

1. A MODULATION EXPANDER FOR A TRANSVERSE-MODE, BEAM TYPE PARAMETRIC AMPLIFIER IN WHICH ELECTRONS OF A BEAM PROJECTED ALONG A PREDETERMINED AXIS TRAVERSE A HELICAL PATH OF A PREDETERMINED PITCH AND WITH A RADIUS REPRESENTING THE AMPLITUDE OF A SIGNAL CONVEYED BY SAID BEAM, SAID EXPANDER COMPRISING: A MULTI-FILAR FIELD GENERATING STRUCTURE ARRANGED COAXIALLY OF SAID BEAM WITH SAID STRUCTURE HAVING A PLURALITY OF PAIRS OF SPACE-OPPOSED CONDUCTIVE ELEMENTS INDIVIDUALLY SKEWED TO REPRESENT A HELIX WHICH IS CO-DIRECTIONAL WITH AND OF THE SAME PITCH AS SAID HELICAL PATH OF SAID ELECTRONS AND SAID STRUCTURE HAVING AN AXIAL LENGTH CORRESPONGING TO AT LEAST ONE COMPLETE CONVOLUTION OF SAID HELIX AND WITH SAID CONDUCTIVE ELEMENTS INDIVIDUALLY BEING GENERALLY OF U-SHAPED CROSS-SECTION AND MUTALLY ORIENTED TO SPACE ADJACENT LEGS IN SIDE-BY-SIDE ALIGNED, PARALLEL AND ELECTRICALLY INSULATED RELATIONSHIP AND TO DISPOSE THE BIGHT PORTIONS NEAREST TO AND FACING SAID AXIS; AND A UNDIRECTIONAL SOURCE OF POTENTIAL CONNECTED WITH ONE POLARITY TO THE EVEN-NUMBERED PAIRS OF SAID CONDUCTIVE ELEMENTS OF SAID STRUCTURE AND CONNECTED WITH OPPOSED POLARITY TO THE ODD-NUMBERED PAIRS OF SAID CONDUCTIVE ELEMENTS TO ESTABLISH WITHIN SAID EXPANDER A PUMPING FIELD WHICH IS UNDIRECTIONAL IN A TIME DOMAIN BUT WHICH EXHIBITS POLARITY REVERSALS AT SPACED POSITIONS ALONG SAID AXIS TO SIMULATE A SYMMETRICAL, NON-HOMOGENEOUS PUMPING FIELD ROTATING IN SYNCHRONISM WITH SAID ELECTRONS TO EXPAND THE RADIUS OF THEIR ORBITAL MOTION. 